Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by a gas. Because the volume of a gas is so much greater than the volume of a liquid containing the same number of gas molecules, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, this contributes to the final cost of the natural gas.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline and middle distillates have been decreasing and supplies are not expected to meet demand in the coming years. Middle distillates typically include heating oil, jet fuel, diesel fuel, and kerosene. Fuels that are liquid under standard atmospheric conditions have the advantage that in addition to their value, they can be transported more easily in a pipeline than natural gas, since they do not require energy, equipment, and expense required for liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen and/or steam to form synthesis gas or syngas, which is a combination of carbon monoxide gas and hydrogen gas. The second transformation is commonly known as the Fischer-Tropsch synthesis, in which carbon monoxide is reacted with hydrogen to form organic molecules containing carbon and hydrogen, such as hydrocarbons and oxygenates (containing oxygen in addition to carbon and hydrogen). Synthesized hydrocarbons generally include paraffins and/or olefins. Paraffins are particularly desirable as the basis of synthetic diesel fuel.
Typically the Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms, and thus having a range of molecular weights. Thus, the Fischer-Tropsch products produced by conversion of natural gas commonly contain a range of hydrocarbons including gases, liquids and waxes. Depending on the molecular weight product distribution, different Fischer-Tropsch product mixtures are ideally suited to different uses. For example, Fischer-Tropsch product mixtures containing liquids may be processed to yield gasoline, as well as heavier middle distillates. Hydrocarbon waxes may be subjected to an additional processing step for conversion to liquid and/or gaseous hydrocarbons. Thus, in the production of a Fischer-Tropsch product stream for processing to a fuel it is desirable to maximize the production of high value liquid hydrocarbons, such as hydrocarbons with at least 5 carbon atoms per hydrocarbon molecule (C5+ hydrocarbons).
The Fischer-Tropsch process is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. A feed containing carbon monoxide and hydrogen is typically contacted with a catalyst in a reaction zone that may include one or more reactors.
Common reactors include packed bed (also termed fixed bed) reactors, fluidized bed reactors and slurry bed reactors. Originally, the Fischer-Tropsch synthesis was carried out in packed bed reactors. These reactors have several drawbacks, such as temperature control, that can be overcome by gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated multiphase reactors sometimes called “slurry reactors” or “slurry bubble columns,” operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are typically converted to gaseous and liquid products. The gaseous products formed enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the suspending liquid by using different techniques like filtration, settling, hydrocyclones, magnetic techniques, etc. Gas-agitated multiphase reactors or slurry bubble column reactors (SBCRs) inherently have very high heat transfer rates, and therefore, reduced reactor cost. This, and the ability to remove and add catalyst online are some of the principal advantages of such reactors as applied to the exothermic Fischer-Tropsch synthesis. Sie and Krishna (Applied Catalysis A: General 1999, 186, p. 55), incorporated herein by reference in its entirety, give a history of the development of various Fischer Tropsch reactors.
Typically, in the Fischer-Tropsch synthesis, the distribution of weights that is observed such as for C5+ hydrocarbons, can be described by likening the Fischer-Tropsch reaction to a polymerization reaction with an Anderson-Shultz-Flory chain growth probability (α) that is independent of the number of carbon atoms in the lengthening molecule. α is typically interpreted as the ratio of the mole fraction of Cn+1 product to the mole fraction of Cn product. A value of α of at least 0.72 is preferred for producing high carbon-length hydrocarbons, such as those of diesel fractions.
The composition of a catalyst influences the relative amounts of hydrocarbons obtained from a Fischer-Tropsch catalytic process. Common catalysts for use in the Fischer-Tropsch process contain at least one metal from Groups 8, 9, or 10 of the Periodic Table (in the new IUPAC notation, which is used throughout the present specification).
Cobalt metal is particularly desirable in catalysts used in converting natural gas to heavy hydrocarbons suitable for the production of diesel fuel. Alternatively, iron, nickel, and ruthenium have been used in Fischer-Tropsch catalysts. Nickel catalysts favor termination and are useful for aiding the selective production of methane from syngas. Iron has the advantage of being readily available and relatively inexpensive but the disadvantage of a high water-gas shift activity. Ruthenium has the advantage of high activity but is quite expensive.
Many petroleum and chemical processes use particulate catalysts for the conversion of a feedstock to one or more desired products, such as the Fischer-Tropsch process described herein. In any reaction requiring a particulate catalyst, the particulate catalyst can be expected to have a certain life, for example several months to a few years. Accordingly, as the time on line increases, the particulate catalyst tends to degrade and eventually becomes ineffective. The catalyst can then be considered ‘spent’ and, a portion of or all of the spent catalyst can be removed from a reactor vessel. In order to maintain catalyst inventory in the reactor, new and/or regenerated catalyst can be loaded therein. The selection of a particulate catalyst composition to be used in a specific reaction system may largely depend on the cost of manufacture of the catalyst and the ability for the catalyst activity to be restored. The spent catalyst removed from the reactor vessel can undergo a regeneration process if the activity of the removed spent catalyst can at least be partially restored. However, in some cases the loss of catalyst activity is irreversible. In this case, it may be possible for the spent catalyst to undergo a reclamation process to recover some of or all of the valuable materials (especially its metallic components).
An important aspect of the economics of a commercial hydrocarbon synthesis process, such as employing the Fischer-Tropsch synthesis, is the efficient utilization of the metals used in the hydrocarbon synthesis particulate catalyst. The initial costs of the metals employed in such particulate catalyst are a large capital expenditure. Accordingly, once the catalyst has, in one way or another, become unacceptable for continued use in the hydrocarbon synthesis process, it is economically wise to take steps to recover the metals from the spent catalyst for reuse, for example to prepare new batches of fresh particulate catalyst. While such reclamation facilities may be located at or in proximity to the hydrocarbon synthesis process site, these facilities are often offsite thus requiring that the spent catalyst be transported or shipped for further processing to extract valuable metallic components. One step in this effort is to prepare the spent catalyst for safe transport to the location where at least some of the metallic components (e.g., metal or metals) will be recovered.
In such cases, it is preferable to remove any residual hydrocarbon products from the spent particulate catalyst prior to processing the spent particulate catalyst through a reclamation process. In this way, one can recover the value of the hydrocarbon products, convert the catalyst from a reactive form to an inert form, avoid the additional transportation costs associated with the weight of the residual hydrocarbon products, and in some instances, minimize the presence of hydrocarbonaceous compounds in any waste materials for environmental conservation reasons.
Therefore, a need exists in the art for efficient methods and systems for the removal of organic materials from solid catalysts, and in particular removal of hydrocarbon products from Fischer-Tropsch spent catalysts, and for the conversion of metallic component(s) of these catalysts into non-reactive form, to lessen safety concerns during transportation to reclamation facilities and/or to facilitate the reclamation of metallic components from such catalysts.